Method for configuring a device to include a negative differential resistance (NDR) characteristic

ABSTRACT

A process for forming/configuring a device to include a negative differential resistance (NDR) characteristic is disclosed. In a FET embodiment, an NDR characteristic is implemented by incorporating a dynamic threshold voltage in such device. An onset point for the NDR characteristic is also adjustable during a manufacturing process to enhance the performance of an NDR device.

RELATED APPLICATIONS

The present invention claims priority to and is a divisional of thefollowing prior application by the present applicant:

(1) an application titled “CMOS COMPATIBLE PROCESS FOR MAKING A TUNABLENEGATIVE DIFFERENTIAL RESISTANCE (NDR) DEVICE ” (Ser. No. 09/602,658filed Jun. 22, 2000); and is further related to and claims priority tothe following applications:

(2) an application tided “CMOS PROCESS COMPATIBLE, TUNABLE NDR (NEGATIVEDIFFERENTIAL RESISTANCE) DEVICE AND METHOD OF OPERATING SAME,” (Ser. No.09/603,101 filed Jun. 22, 2000); and

(3) an application tided “CHARGE TRAPPING DEVICE AND METHOD FORIMPLEMENTING A TRANSISTOR HAVING A NEGATIVE DIFFERENTIAL RESISTANCEMODE” (Ser. No. 09/603,102 filed Jun. 22, 2000).

The above materials are expressly incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a process for making a device that exhibitsnegative differential resistance. The present invention is applicable toa wide range of semiconductor integrated circuits, particularly forhigh-density memory and logic applications, as well as power managementdevices.

BACKGROUND OF THE INVENTION

Devices that exhibit a negative differential resistance (NDR)characteristic, such that two stable voltage states exist for a givencurrent level, have long been sought after in the history ofsemiconductor devices. When Nobel Prize winner Leo Esaki discovered theNDR characteristic in a resonant tunneling diode (RTD), the industrylooked expectantly to the implementation of faster and more efficientcircuits using these devices. NDR based devices and principles arediscussed in a number of references, including the following that arehereby incorporated by reference and identified by bracketed numbers [ ]where appropriate below:

[1] P. Mazumder, S. Kulkarni, M. Bhattacharya, J. P. Sun and G. I.Haddad, “Digital Circuit Applications of Resonant Tunneling Devices,”Proceedings of the IEEE, Vol. 86, No. 4, pp. 664-686, 1998.

[2] W. Takao, U.S. Pat. No. 5,773,996, “Multiple-valued logic circuit”(issued Jun. 30, 1998)

[3] Y. Nakasha and Y. Watanabe, U.S. Pat. No. 5,390,145, “Resonancetunnel diode memory” (issued Feb. 14, 1995)

[4] J. P. A. Van Der Wagt, “Tunneling-Based SRAM,” Proceedings of theIEEE, Vol. 87, No. 4, pp. 571-595, 1999.

[5] R. H. Mathews, J. P. Sage, T. C. L. G. Sollner, S. D. Calawa, C. -L.Chen, L. J. Mahoney, P. A. Maki and K. M Molvar, “A New RTD-FET LogicFamily,” Proceedings of the IEEE, Vol. 87, No. 4, pp. 596-605, 1999.

[6] H. J. De Los Santos, U.S. Pat. No. 5,883,549, “Bipolar junctiontransistor (BJT)—resonant tunneling diode (RTD) oscillator circuit andmethod (issued Mar. 16/1999)

[7] S. L. Rommel, T. E. Dillon, M. W. Dashiell, H. Feng, J. Kolodzey, P.R. Berger, P. E. Thompson, K. D. Hobart, R. Lake, A. C. Seabaugh, G.Klimeck and D. K. Blanks, “Room temperature operation of epitaxiallygrown Si/Si_(0.5)Ge_(0.5)/Si resonant interband tunneling diodes,”Applied Physics Letters, Vol. 73, No. 15, pp. 2191-2193, 1998.

[8] S. J. Koester, K. Ismail, K. Y. Lee and J. O. Chu, “Negativedifferential conductance in lateral double-barrier transistorsfabricated in strained Si quantum wells,” Applied Physics Letters, Vol.70, No. 18, pp. 2422-2424, 1997.

[9] G. I. Haddad, U. K. Reddy, J. P. Sun and R. K. Mains, “Thebound-state resonant tunneling transistor (BSRTT): Fabrication, d.c. I-Vcharacteristics, and high-frequency properties,” Superlattices andMicrostructures, Vol. 7, No. 4, p. 369, 1990.

[10] Kulkarni et. al., U.S. Pat. No. 5,903,170, “Digital Logic DesignUsing Negative Differential Resistance Diodes and Field-EffectTransistors (issued May 11, 1999).

A wide range of circuit applications for NDR devices are proposed in theabove references, including multi-valued logic circuits [1,2], staticmemory (SRAM) cells [3,4], latches [5], and oscillators [6]. To date,technological obstacles have hindered the widespread use of RTD devicesin conventional silicon-based integrated circuits (ICs), however.

The most significant obstacle to large-scale commercialization has beenthe technological challenge of integrating high-performance NDR devicesinto a conventional IC fabrication process. The majority of RTD-basedcircuits require the use of transistors, so the monolithic integrationof NDR devices with predominant complementary metal-oxide-semiconductor(CMOS) transistors is the ultimate goal for boosting circuitfunctionality and/or speed. Clearly, the development of aCMOS-compatible NDR device technology would constitute a break-throughadvancement in silicon-based IC technology. The integration of NDRdevices with CMOS devices would provide a number of benefits includingat least the following for logic and memory circuits:

1) reduced circuit complexity for implementing a given function;

2) lower-power operation; and

3) higher-speed operation.

Significant manufacturing cost savings could be achieved concomitantly,because more chips could be fabricated on a single silicon wafer withouta significant increase in wafer-processing cost. Furthermore, a CMOScompatible NDR device could also be greatly utilized in power managementcircuitry for ICs, which is an area of growing importance due to theproliferation of portable electronic devices (PDAs, cell phones, etc.)

A tremendous amount of effort has been expended over the past severaldecades to research and develop silicon-based NDR devices in order toachieve compatibility with mainstream CMOS technology, because of thepromise such devices hold for increasing IC performance andfunctionality. Efforts thus far have only yieldedquantum-mechanical-tunneling-based devices that require eitherprohibitively expensive process technology or extremely low operatingtemperatures which are impractical for high-volume applications. Onesuch example in the prior art requires deposition of alternating layersof silicon and silicon-germanium alloy materials using molecular beamepitaxy (MBE) to achieve monolayer precision to fabricate the NDR device[7]. MBE is an expensive process which cannot be practically employedfor high-volume production of semiconductor devices. Another example inthe prior art requires the operation of a device at extremely lowtemperatures (1.4K) in order to achieve significant NDR characteristics[8]. This is impractical to implement for high-volume consumerelectronics applications.

A further drawback of the tunnel diode is that it is inherently atwo-terminal device. Three (or more) terminal devices are preferred asswitching devices, because they allow for the conductivity between twoterminals to be controlled by a voltage or current applied to a thirdterminal, an attractive feature for circuit design as it allows an extradegree of freedom and control in circuit designs. Three-terminal quantumdevices which exhibit NDR characteristics such as the resonant tunnelingtransistor (RTT) [9] have been demonstrated; the performance of thesedevices has also been limited due to difficulties in fabrication,however. Some bipolar devices (such as SCRs) also can exhibit an NDReffect, but this is limited to embodiments where the effect is achievedwith two different current levels. In other words, the I-V curve of thistype of device is not extremely useful because it does not have twostable voltage states for a given current.

Accordingly, there exists a significant need for a new three-terminalNDR device which can be easily and reliably implemented in aconventional CMOS technology. In addition, it is further desirable thatsuch a three-terminal device can be operated at room temperature.

One useful observation made by the inventors concerning an ideal NDRdevice is to notice that its I-V curve looks essentially like that of anon-volatile memory cell that has a dynamic and reversible thresholdvoltage. The inventors thus noted that if a non-volatile memory could becontrolled in this fashion, it might be possible to achieve an NDReffect. To date, however, the inventors are unaware of anyone succeedingwith or even attempting such an approach. For example, in a prior artdevice described in U.S. Pat. No. 5,633,178, and incorporated byreference herein, a type of volatile memory device is depicted, in whichelectrons are stored in charge traps near a substrate/dielectric layerinterface. Notably, this reference discusses the filling and emptying ofthe traps through programming operations (to store a 0 or 1), but doesnot identify any implementation or variation that is suitable for an NDRapplication, or which even suggests that it is capable of dynamic orquickly reversible threshold voltage operation. Similar prior artreferences also identify the use of charge traps for non-volatilememories, but none again apparently recognize the potential use for suchstructures in an NDR context. See, e.g., U.S. Pat. Nos. 4,047,974;4,143,393; 5,162,880 and 5,357,134 incorporated by reference herein.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a new type ofsemiconductor device, which like the tunnel diode, exhibits a negativedifferential resistance (NDR) characteristic that can be utilized todramatically improve the performance and functionality of integratedcircuits;

Another object of the present invention is to provide a new NDR devicein which band-to-band tunneling is not the sole physical mechanismresponsible for the negative differential resistance characteristic;

Another object of the present invention is to provide a new device inwhich charge trapping can be used for achieving a negative differentialresistance characteristic;

Yet another object of the present invention is to provide a new NDRdevice with full transistor features (i.e., a three-terminal device),where the conductivity between two terminals is controlled by a voltageor current applied to the third terminal;

Yet another object of the present invention is to provide a new NDRdevice which can be fabricated with a process that is fully compatiblewith conventional CMOS process technology;

Yet another object of the present invention is to provide a new NDRdevice whose lateral dimensions can scale in proportion with the scalingof CMOS devices;

Yet another object of the present invention is provide a new NDR devicewhere the voltage corresponding to the onset of negative differentialresistance is fully tunable;

Yet another object of the present invention is to provide a new devicewhere the peak current as well as the negative differential resistancebetween two terminals can be tailored by adjusting the voltage appliedto a third terminal;

Finally, another object of the present invention is to provide a devicethat will be useful for power management applications in portableelectronic devices, including as a voltage regulator, an overcurrentprotection device, etc.

These and other objects are achieved by the present invention thatdiscloses a new NDR transistor that can be implemented usingconventional integrated-circuit process technology. The new deviceoffers significant advantages over prior art: an electronically tunableNDR; extremely high peak-to-valley current ratio (greater than 1000 forroom-temperature operation); compatibility with conventional CMOSprocess technology; and scalability to future generations of CMOSintegrated-circuit technology.

A first aspect of the invention concerns a semiconductor transistordevice that achieves a negative differential resistance mode by using adynamically variable and reversible threshold voltage. The thresholdvoltage can be dynamically controlled using a conventional gate controlsignal. Unlike prior art devices, the negative differential resistanceis based on temporary charge trapping/detrapping mechanism, and not on aband-to-band tunneling mechanism.

Another aspect of the invention pertains to a semiconductor transistordevice which has three control terminals, and is operable with anegative differential resistance mode by applying a bias signal acrosstwo of the terminals to set up a current path between the two terminals,and a control signal to a separate third terminal for controllingconduction in the current path by controlling a density of chargecarriers available in the current path.

Another aspect of the invention concerns a single charge carriersemiconductor device which is operable with a negative differentialresistance mode as noted above with two stable voltage states, and isfabricated using only complementary metal oxide semiconductor (CMOS)processing.

A further aspect of the invention pertains to a dielectric trappinglayer located proximate to a transistor channel. The transistor channelis capable of carrying a current that varies from a first current valueassociated with a conducting condition for the transistor channel, to asecond current value associated with a non-conducting condition for thetransistor channel channel, the second current value being substantiallyless than the first current value. A plurality of carrier trapping siteswithin the dielectric layer are configured for trapping carriers thatare electrically biased by an electrical control field to move from thechannel into the dielectric layer. A negative differential resistancemode can be caused in the channel by rapid trapping and de-trapping ofelectrons to and from the charge trapping sites.

The trapping sites have a concentration and arrangement within thedielectric layer so that the current in the transistor channel can bevaried between the first current value and the second current value bythe action of the trapping sites adjusting the current in accordancewith a value of the electrical control field, and such that thetransistor channel exhibits negative differential resistance. This isdue to the fact that a field generated by the carriers stored in thetrapping layer can be adjusted to be sufficiently large so as to causethe channel to be depleted of carriers, thus reducing the current in thechannel even as the channel bias voltage is increased, and dynamicallyincreasing a threshold voltage of an associated FET.

Other more detailed aspects of the trapping layer and trapping sitesinclude the fact that the trapping sites are located very close (within1.5 nm preferably) to the channel/trapping layer interface. Furthermore,the trapping and detrapping time of the electrical charges can becontrolled through the placement and concentration of the trappingsites. In this fashion, a device can exhibit anything from veryshort/temporary storage times to very long storage times so that auseful substitute can be realized for a non-volatile floating gate typestructure. This type of embedded, spatially distributed electrode of thepresent invention can exhibit substantial operating advantages overconventional single layer, continuous type electrodes commonly used innon-volatile memories.

Another aspect of the invention relates to the fact that the trappinglayer is used in connection with a FET so that in a first operatingregion for the FET the source-drain current has a value that increasesas the lateral electrical field between the source and drain increases,and in a second operating region for the semiconductor device thesource-drain current has a value that decreases as the electrical fieldincreases. Accordingly the drain region and the gate are controlled sothat the device constitutes a three terminal device that can be operatedin a range that exhibits negative differential resistance, because thecharge trapping sites in the gate dielectric serve to trap electrons,causing the FET threshold voltage to increase dynamically, therebyreducing an output current of the FET as a drain-to-source voltagedifference is increased. The trapping and de-trapping actions are alsocontrolled so that they do not occur primarily near a drain junction ofthe FET. Other more detailed features of this aspect of the inventioninclude the fact that the drain dopant concentration profile is tailoredto minimize impact ionization current between the drain region and thechannel region as well as to minimize junction capacitance between thedrain region and the semiconductor substrate. Other more detailedfeatures of this aspect of the invention include the fact that thetrapping layer is formed as an integral part of a gate dielectric forthe FET which includes one or more of the following materials:silicon-dioxide, silicon-nitride, and/or silicon-oxynitride, and/or ahigh-permittivity layer with a relative permittivity greater thanapproximately eight (8). Furthermore, this gate dielectric has athickness adapted to mininimize loss of trapped charge due toquantum-mechanical tunneling. When the gate dielectric issilicon-dioxide it can be formed either entirely or partially by thermaloxidation of heavily doped (>10¹⁸ cm⁻³) p-type silicon. The chargetrapping sites thus consist of defects within the silicon-dioxide formedby thermal oxidation of the doped p-type silicon. Alternatively, thecharge trapping sites can consist of islands of metal or semiconductormaterial, or even a floating gate embedded in the gate dielectric.

In other variations, the trapping layer/gate dielectric consists of aplurality of dielectric layers. In such embodiments, the charge trappingsites can consist of defects located near an interface between adjacentlayers of the gate dielectric.

Another aspect of the invention pertains to the fact that the channelcan be subjected to an electrical field having a first field componentalong the surface resulting from a bias voltage applied to the sourceand drain regions, and a second field component substantiallyperpendicular to the surface resulting from a control voltage applied tothe control gate. These field components control how carriers in thechannel acquire sufficient energy to overcome an interface barrierbetween the channel and the trapping layer, and how quickly they aretrapped and detrapped.

Other more detailed aspects of the invention pertaining to the channelcharacteristics include the fact that the energetic (“hot”) carriers aregenerated (and thus trapped) substantially uniformly throughout a lengthof the channel region, instead of being concentrated at a junctioninterface as occurs in the prior art. The channel is also heavily p-typedoped, and has a dopant concentration that peaks near the semiconductorsurface, to enhance the generation of hot electrons. Furthermore, it canbe offset from the source and drain regions to minimize junctioncapacitance.

Other aspects of the present invention relate to methods of operatingthe devices described above.

Finally, other aspects of the present invention relate to methods ofmaking the structures and devices above. These include manufacturingprocesses which are compatible with conventional CMOS techniques used incommercial semiconductor facilities thus providing a substantialadvantage over the prior art. An additional benefit lies in the factthat the onset point for the negative differential resistance mode canbe adjusted during the making of the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the schematic cross-sectional view of an embodiment of the NDRmetal-insulator-semiconductor field-effect transistor (MISFET) disclosedin this invention.

FIG. 2 is a graphical chart illustrating the current versus voltage(I-V) characteristics of the NDR-MISFET, including an NDR operatingregion.

FIG. 3 is the schematic cross-sectional view of another embodiment ofthe NDR-MISFET disclosed in this invention.

FIG. 4 is an illustrative process sequence for integrating theNDR-MISFET into a conventional CMOS logic process flow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description is meant to be illustrative only ofparticular embodiments of the invention. Other embodiments of theinvention and variations of those disclosed will be obvious to thoseskilled in the art in view of the following description.

As discussed below, a preferred device embodiment is described first.Next, the mechanism responsible for the negative differential resistance(NDR) mode is described, followed by additional preferred embodimentsfor enhancing the performance of an NDR device. Finally, an exemplarymethod of fabrication will be described.

In accordance with a preferred embodiment of the invention, an n-channelMISFET NDR device structure (FIG. 1) 100 is provided which is made withminimum modification to a standard CMOS process. In fact, from a firstglance, device 100 appears to be an ordinary n-channel MOS (NMOS)transistor, in which a gate electrode 110 of the device is formed on topof a semiconductor substrate 120 and electrically insulated from thesubstrate by a dielectric layer 130. Right away it can be seen that NDRdevice 100 in this invention is distinctly different from NDR devices inthe prior art.

Prior-art NDR devices are typically two-terminal diode devices, madewith very complicated and expensive process sequences which areincompatible with a conventional CMOS process. Although NDR device 100in this invention is similar in appearance to an NMOS transistor, itincorporates slight but critical modifications, as taught in thisinvention, in order for the device to manifest the desired NDR outputcharacteristic mode.

A first modification is that a p-type dopant concentration in a surfaceregion of the semiconductor substrate underneath the gate electrode (thechannel) is relatively high compared to a contemporary conventionallyprocessed n-channel device. In a preferred embodiment of device 100, thep-type dopant concentration is greater than 1×10¹⁸ cm⁻³ in the channel.Of course, it will be understood that for any particular design rule,device characteristic and process environment the p-type dopantconcentration may be varied accordingly, and that some routine design,simulation and/or testing may be necessary to optimize the performanceof the device in any particular application. Accordingly, the presentinvention is not limited to any particular concentration, but, instead,is guided more by considerations of whether a sufficient dopantconcentration has been introduced to help contribute to the NDR effect.More heavily doped n-type regions in the semiconductor surface region,adjacent to the channel and located at each end of the gate electrode,form the source and drain contact regions 140 and 150 respectively. Theelectric potential of the channel can be further adjusted via a bodycontact terminal 125.

A second modification of present device 100 over a conventionaltransistor is the fact that charge traps or storage nodes 135 exist ininsulating layer 130 between semiconductor substrate 120 and gateelectrode 110. These charge traps are located relatively close to(within 1.5 nm of) semiconductor-insulator interface 138, so thatcharges from semiconductor 120 can be trapped and de-trapped veryquickly. Again it will be understood that this distance figure is basedon the details of the present embodiment, and that for any particularenvironment this parameter may vary significantly, so the presentinvention is not limited by the particular details of the same. The keypoint, of course, is the existence of these charge traps, or some otherphysical feature that acts to store electrons. It will be understood ofcourse that the drawing of FIG. 1 is merely an illustration to betterdescribe the features of the present invention, and thus the arrangementand location of the trapping sites 135 is not drawn to scale. A thirdmodification is that insulating layer 130 between semiconductorsubstrate 120 and gate electrode 110 is relatively thick (greater than 6nm) to prevent significant loss of trapped charge to the gate electrodevia tunneling. Those skilled in the art will again appreciate that thisthickness is again a function of the particular material, processingenvironment, etc., and that the present invention is by no means limitedto such figure.

With source and body terminals 145 and 125 of device 100 held at groundpotential and gate terminal 115 biased sufficiently high to turn on thedevice, the output characteristic (drain current as a function of drainvoltage) of device 100 will exhibit negative differential resistanceover a range of drain voltages. This aspect of the invention isillustrated in FIG. 2, where device drain current versus drain voltageis plotted for two different gate voltages to show how the NDR mode canbe affected by a suitable selection of the gate voltage. It can be seenthat for a fixed gate voltage V_(GS), drain current I_(DS) firstlyincreases in a first region 210 with drain voltage V_(DS), similarly tothe behavior that is seen in drain current in a conventional NMOStransistor. Surprisingly, however, in region 220, beyond a certain drainvoltage level, drain current decreases with further increases involtage, i.e. the device exhibits an NDR mode with NDR characteristics.The drain voltage at which the drain current begins to decrease (i.e.,point 225 where V_(DS)=V_(NDR)) is adjustable through suitableselections of channel length, threshold voltage, etc. It should be notedthat, due to the relatively high channel dopant concentration and therelatively thick gate dielectric, the threshold voltage of the NDR FETwill be significantly higher than that of a conventional MOSFET, so thata larger than typical gate voltage is correspondingly used for the NDRFET. As a result, V_(GS)>V_(NDR) so that the vertical electric field isin the direction such that electrons are attracted towards the gateelectrode, enhancing the NDR effect.

This behavior by device 100 of the present invention is rathersurprising, and is apparently the result of physical mechanisms thathave hitherto not been exploited in this area of semiconductor devicesand processing. In the prior art, band-to-band quantum-mechanicaltunneling of charged particles (electrons and/or holes) from one side ofa diode to the other side is known to be the primary mechanism for NDRin tunneling diodes. In contrast, for device 100 of the presentinvention, the physical mechanism appears to be rapid trapping ofelectrons in the gate insulator underneath the gate electrode, near to(within 1.5 nm of) the semiconductor-insulator interface. Referring tothe device structure in FIG. 1, when device 100 is biased with asufficiently high gate voltage such that the channel of the device is inthe strong-inversion condition (i.e. when the gate-to-source voltage isgreater than the threshold voltage), a current flows between the sourceand drain terminals 145 and 155 respectively of the device if a smallvoltage is applied between such terminals. Since the channel isconfigured to contain a relatively high p-type dopant concentration, avertical (in the direction perpendicular to the semiconductor surface)electric field in the channel is large (greater than 10⁶ V/cm). As thedrain-to-source voltage increases, the lateral (in the directionparallel to the semiconductor surface) electric field increases, so thata composite (horizontal+vertical) electric field exerting force oninversion-layer electrons in the channel increases. Once this compositeelectric field reaches a certain critical value (which of course will bea function of the doping and geometry of the device) electrons flowingfrom source 140 to drain 150 will gain sufficient energy betweencollisions to surmount a semiconductor-insulator interface potentialbarrier. Since the vertical electric field component attracts theelectrons toward gate electrode 110, electrons enter insulator 130 andsubsequently are captured by the traps or storage nodes 135 in theinsulator. The presence and accumulation of negative charge in insulator130 dynamically increases a threshold voltage of device 100. In otherwords, the electrons accumulated in the traps/storage nodes 135 operateto set up a counter field that inhibits the movement of additionalelectrons into the channel from the source, and reducing an availablechannel current by reducing a density of electrons in the channelregion. Thus, the net effect created by the traps/storage nodes 135 ofthe present invention is a drastic reduction in the inversion-layercharge density and commensurate reduction in the current flowing betweenthe source and the drain. It can be seen plainly that the amount of netcurrent in the channel that can be affected by the traps is a functionof their number, concentration, location, and the bias conditionsimposed on device 100, all of which are easily controllable andoptimizable for any particular environment, so that the onsetconditions, strength and operating region for a negative differentialresistance mode can be tailored and customized as needed.

It is noted that the present disclosure teaches that only a singlespecies of energetic carriers (hot electrons) are generated in a channelregion and trapped in insulator 130, and both of these phenomenapreferably occur in a substantially uniform manner throughout thechannel length. This operation, too, is distinctly different from thecase for a conventional NMOS transistor, in which hot electrons aregenerally generated in the depletion region of the drain p-n junction,leading to impact ionization and an avalanche effect resulting insignificant numbers of hot holes as well as hot electrons. Typically,this effect is maximized at a gate-to-source voltage which is lower thanthe drain-to-source voltage (for example, at a gate voltage equal to onehalf the drain voltage); hence in a conventional device the verticalelectric field in the channel near the drain junction attracts hotholes, rather than hot electrons, toward the gate electrode. Clearly,then, this explains why the creation of hot electrons in a conventionalNMOS transistor (even if it occurs incidentally) cannot produce thenegative differential resistance characteristic as described in thisinvention. Furthermore it is well known that the injection of hot holesinto the gate insulator causes damage, adversely affecting theperformance and reliability of the NMOS transistor. In the NDR-MISFET100 of the present invention, although holes are generated by impactionization in the channel, they are not injected (or their injection issubstantially eliminated to the point where it is negligible from anoperational perspective) into gate insulator 130 because the verticalelectric field repels holes from gate electrode 110.

As a point of further clarification, the mechanism responsible for theNDR characteristic of the present invention also does not require thatNDR MISFET 100 be operating in a conventional “pinch-off” condition,i.e., in which a gate-to-drain voltage is lower than a threshold voltageso that the inversion-layer charge density in the channel adjacent tothe drain is zero. In the pinch-off condition, the lateral electricfield is non-uniformly distributed in the channel between the source anddrain: the electric field increases gradually and linearly with distanceaway from the source, and then increases exponentially in the depletionregion of the drain junction, so that the generation of hot electronsoccurs predominantly in the depletion region of the drain junction,resulting in drain avalanche. In contrast, in the present invention,NDR-MISFET 100 is preferably operated in a “triode” region, so that theelectric field increases uniformly from the source end of the channel tothe drain end. The drain current saturates due to velocity saturation,not pinch-off, so the current does not increase linearly with V_(DS) (asseen generally in FIG. 2).

In a preferred embodiment of NDR-MISFET 100, sufficient bias is appliedso that the electrons in the channel become so energetic that channelhot electrons are created due to the high composite electric field inthe channel. These channel hot electrons have sufficient energy impartedfrom the horizontal component of this field to surmount the potentialbarrier at the semiconductor-insulator interface and enter gateinsulator 130 because the vertical electric field component attractsthem toward gate electrode 110. The electrons are captured by the trapsor storage nodes 135 in insulator 130; consequently the thresholdvoltage of the transistor increases dynamically. More charge is trappedas the drain-to-source voltage increases (for a constant gate voltage),because the generation of hot carriers (and thus the percentage of thecurrent that is based on a hot carrier component) correspondinglyincreases, and it is these hot carriers that are trapped. As greaternumbers of hot carriers are trapped, they increase the threshold voltageand thereby reduce the mobile charge density in the channel by adisproportionate amount (compared to the hot-carrier current chargeamount), thus decreasing the drain current dramatically. This results inthe negative differential resistance in the output (drain current versusdrain voltage) characteristic. It can be seen also that more charge canbe trapped by increasing the vertical component of the field as well,since this increases the likelihood that a charged carrier will beforced into a trap 135 in dielectric layer 130 (the trapping rate), andalso increases a temporary storage/trapping time associated with thecharge. It is not necessary, nonetheless, to trap a significant numberof carriers, because even a small quantity stored in the trapping sitescan be sufficient to deplete the channel of mobile carriers. It is alsopreferable to not increase the vertical field to the point where somedeleterious side effects (dielectric breakdown or lack of fastreversibility of the NDR effect for example) are seen. In other words,it is generally desirable to have the charges rapidly trapped andde-trapped at a particular rate that ensures that the device can be putinto and out of an NDR mode or operating region quickly, instead ofbeing confined to working within a particular region. Other techniquesfor increasing the amount of trapped charges, and thetrapping/detrapping rates will be apparent to those skilled in the art.For instance, it may not be necessary in fact in some applications, tomake the electrons “hot” because they will still be swept by thevertical field into the trapping sites.

Thus, the present invention uses an approach that is in contrast to thatof prior art which has charge traps, such as U.S. Pat. No. 5,633,178. Inthe prior art, the emphasis has been on retaining the charge as long aspossible, and this reference for example specifically discloses using arefresh operation to keep the logic state. Accordingly, there is noeffort made in the prior art to implement or sustain a dynamic processwhere charges are continually trapped and de-trapped. In factconventional disclosures discourage such condition because it has beenperceived to date as an undesirable situation, and so this explains,too, why such references do not describe configuring a FET channel tohave a structure and doping characteristics that would facilitate thistype of trapping/detrapping mechanism.

The drain current and therefore the negative differential resistance inthis invention can be adjusted by varying the gate voltage as seen inFIG. 2. As seen also in FIG. 2, the invention can be seen as exploitingthe fact that, as the threshold voltage V_(t) dynamically increases(because of the accumulation of trapped charges) with increasingdrain-to-source voltage V_(DS), a drain current I_(DS) (which isproportional to V_(g)−V_(t)) will first increase, and then begin todecrease as V_(t) begins to exceed V_(g) and thus dominate the behaviorof the device. Thus, a current value depicted in curve 228 willgenerally follow the set of continuous curves 229 shown in FIG. 2 for agiven V_(g) and varying V_(t). The so-called “peak-to-valley ratio,” akey figure of merit in NDR devices, can also be tuned in the presentinvention through suitable combinations of doping concentrations, devicegeometries and applied voltages.

The present invention bears some resemblance to a leaky (or volatile)floating gate storage device. However, the trapping and de-trapping ofelectrons in gate insulator 130 of NDR-MISFET 100 are very rapidprocesses, as compared to the programming and erase processes of aconventional floating-gate non-volatile memory device, so that thethreshold voltage of NDR-MISFET 100 can respond dynamically to changesin a gate-to-source voltage and/or a drain-to-source voltage. In fact,while conventional memory devices require extensive pre-programming anderase cycle times to change threshold states, the threshold voltage ofthe present device responds to the applied source to drain bias voltagewith minimal delay. Thus, it can change and reverse a threshold (andthus achieve an NDR mode) in substantially the same time as it takes fordevice 100 to turn the channel on or off in response to such biasconditions. For any given bias condition (fixed gate-to-source anddrain-to-source voltages), a steady-state condition exists in whichelectrons are continually being rapidly trapped, stored, and de-trapped,maintaining a fixed amount of net charge trapped in gate insulator 130.The fixed amount of net charge trapped in the gate insulator isdependent on the particular voltage bias conditions applied to device100. As the gate-to-source voltage and/or the drain-to-source voltagechanges, the balance of the trapping and de-trapping processes changes,thereby changing the fixed amount of net charge trapped in the gateinsulator and dynamically changing the threshold voltage. This means thenet NDR effect can be controlled through two different bias parameters,a significant advantage again over conventional two terminal NDRdevices. Furthermore, the negative differential resistancecharacteristic is seen not only as the drain-to-source voltage isincreased from zero Volts to a high value (such that hot electrons aretrapped in gate insulator 130), but also in the reverse direction as thedrain-to-source voltage is decreased from a high value to zero Volts. Itis expected, in fact that the threshold voltagevariability/reversibility can be tailored to be relatively symmetric, sothat it can thus be adjusted from a relatively low voltage value to arelatively high voltage value in approximately the same time required toadjust the threshold voltage from a relatively high voltage value to arelatively low voltage value.

As intimated above, the inventors believe that at higher drain to sourcevoltages another feature of the present invention will be apparent, andthat is the relatively high percentage of hot carriers in the channelcurrent. Namely, since hot carriers are generated at a faster rate asthe drain to source voltage increases the inventors believe that the netresult is that eventually the hot carrier current component of thechannel current will become dominant, and thus eventually constitute theonly current component in the channel, even if it is extremely smalloverall. The relative percentage of hot carriers in the channel current,therefore, can be controlled, and this feature of the invention may bebeneficial in other application environments.

Another aspect of the invention that is potentially useful is the factthat the trapping sites of the present invention can be thought of asintroducing a form of current/charge delay on a single channel basis.The trapping time, temporary storage time, and detrapping time making upsuch delay can be controlled as a function of the applied horizontal andvertical electrical fields, and this aspect might be exploited in otherenvironments.

As explained herein, the p-type dopant concentration in the surfaceregion of the semiconductor underneath the gate electrode should berelatively high. This is to ensure that the vertical electric field ishigh (greater than 10⁶ V/cm) when the transistor is turned on, topromote the creation of hot electrons in the channel. A conventionalNMOS transistor with channel length less than 250 nm may (in someapplications) have such a high channel dopant concentration, but it willnot achieve the results of the present invention because this structurealone is insufficient to bring about an NDR effect. In a preferredembodiment, the doping concentration is made slightly graded, so thatthe concentration of dopant is slightly lower at the semiconductorsurface, and then peaks at some relatively small distance (below 30 nm)below the surface. This is done in order to achieve a built-in electricfield, which in turn serves to confine electrons near the surface of thesemiconductor, and thus further enhances the injection of electrons intothe trapping sites in the dielectric. Again, other doping concentrationsand techniques can also be employed to induce this same phenomenon.

Furthermore, to minimize the possibility of drain avalanche, a preferredembodiment herein teaches that the drain dopant-concentration profile atthe junction with the channel is made to be relatively lightly doped.This not only minimizes the impact ionization current between the drainand the channel, but also has the side benefit of minimizing thecapacitance between them. By minimizing the drain junction capacitanceto the channel, the overall device switching performance is enhanced andthe device thus operates faster. Those skilled in the art willappreciate that there are other ways to enhance the generation of hotelectrons in the channel in addition to those described herein, and thepresent invention is not limited to any particular implementation of thesame.

A preferred embodiment also confines the relatively high dopantconcentration in the channel to the surface region only, so that thedopant concentration in the channel region is initially low (to confineelectrons to the surface region), then increases, and then is made loweraway from the surface to achieve the effect of low drain-junctioncapacitance. As alluded to earlier, the present invention is not limitedto any particular doping concentration and profile of the dopant in thechannel, because the range of such parameters necessary to bring aboutthe NDR effect will vary from device to device of course, depending onthe size, geometry, intended function, etc., of the device, but thesedetails can be gleaned with routine and conventional simulation andtestings for any particular application, in the same manner as is donefor any other conventional semiconductor device. As explainedpreviously, the high surface dopant concentration in the channel shouldalso be offset from the highest dopant concentration in drain region 150through the use of lightly doped drain (LDD) structures.

One additional and very desirable feature of the present invention isthat the drain voltage at the onset of negative differential resistancecan be scaled with the scaling of the CMOS technology. In other words,as the transistor channel length is reduced, the drain voltage requiredto reach the critical composite electric field in the channel(corresponding to the onset of negative differential resistance) iscommensurately reduced. This aspect of the invention ensures that thestructures and methods taught herein are guaranteed to have substantialand meaningful future utility in advanced generations of devices andproducts that are made using smaller geometries, lower bias conditions,etc. than those currently available.

As is evident, a key feature of NDR-MISFET 100 is that charge traps orstorage nodes 135 exist in gate insulator 130, very near to (within 1.5nm of) the semiconductor-insulator interface, so that electrons can betrapped and de-trapped very quickly. The creation anddistribution/location of such traps 135 can be accomplished in anynumber of ways that are compatible with conventional semiconductorprocessing techniques. For example, traps 135 can consist of defectsites within gate dielectric 130 as shown in FIG. 1, or interfacialtraps 135 between two or more layers of a multi-layered gate-insulatorstack, or one or more electrically isolated (“floating”) conductor orsemiconductor electrodes 137 embedded within a gate insulator 130 (madeup of two layers 130′ and 130″ sandwiching the embedded electrode 137)as shown in FIG. 3. The only important consideration is that the carriertrapping sites are configured for trapping carriers that areelectrically biased by an electrical control field (i.e., the combinedeffect of bias conditions resulting from the channel doping, the gate tosource voltage, the source to drain voltage) to move from the channelinto insulator/dielectric layer 130. This can be done in any number ofdifferent concentrations and arrangements within layer 130 so that thechannel current can be varied all the way from essentially zero (noconduction) to full conduction in accordance with the strength of theelectrical control field.

In a preferred embodiment of the present invention, Boron atomsincorporated into gate insulator 130 during a thermaroxidation ofheavily boron-doped silicon serve to provide defect sites which readilytrap charge. Alternative embodiments may employ alternative dopantspecies such as Indium to form charge traps 135, and the presentinvention is not limited to any particular dopant species in thisregard.

As mentioned, other possible embodiments may employ a multi-layered gateinsulator, for example a very thin interfacial layer of silicon dioxideand a thicker layer of a second dielectric material such as siliconnitride, with charge-trapping sites at the dielectric-dielectricinterface. Further possible embodiments may incorporate islands ofmetal, silicon or germanium nanocrystals embedded within gate insulator,or perhaps even a single continuous floating gate electrode (FIG. 3)137, to trap charge. In fact, the present approach can be taken to anextreme to effectuate a new type of non-volatile floating gate electrodefor a flash memory cell. It can be seen that complete non-volatility canbe achieved by simply locating the trapping sites sufficiently far awayfrom the interface so that the charge does not leak off after it is putthere (using conventional programming techniques). This type ofdiscontinuous floating gate electrode, formed as a multitude of trappingsites distributed in the gate dielectric, may have significant operatingadvantages over conventional continuous electrode. In particular, in thedistributed charge storage sites aspect of the present invention, thetrapped charge has less mobility than an electron in a sheet typeelectrode, and thus the charge storage sites are less likely to leak thestored charge (individually and in the aggregate of course) to thesource/drain regions. This in turn means that the charge storage sitescan be located closer to the channel, and thus the gate insulating layercan be thinner, the programming voltage and/or current smaller, etc.,Other methods and techniques for creating and distributing traps 135 ina fashion suitable for achieving an NDR effect, and any non-volatileeffects as shown herein will be apparent to those skilled in the artfrom the present teachings, and can be further gleaned from thedescriptions given in the aforementioned prior art references forcreating different types and arrangements of charge traps.

To enhance the electron trapping stemming from the generation of hotelectrons in the channel (since it is the primary mechanism responsiblefor the negative differential resistance characteristic) the presentdisclosure also teaches a preferred embodiment of an insulator 130 forretaining the trapped charge under high gate-voltage bias. To avoid theloss of trapped electrons to gate electrode 110 via tunneling throughgate insulator 130, the latter should have sufficient thickness toprevent or at least substantially reduce such tunneling effects. In apreferred embodiment insulator 130 is silicon dioxide formed by eitherone of, or a combination of conventional thermal oxidation anddeposition techniques. As referred to earlier, to avoid significant lossof trapped charge due to quantum-mechanical tunneling, gate insulator130 is formed to have a thickness of at least 6 nm. Otherimplementations of insulator material for layer 130 include SiliconNitride (Si₃N₄), or Silicon Oxynitride (SiO_(x)N_(y)), or ahigh-permittivity dielectric (relative permittivity greater than 8). Theuse of a high-permittivity gate dielectric is advantageous for achievinghigh areal gate capacitance, which facilitates adequate gate control ofthe channel potential. Again, the present invention is not restricted toany particular selection of thickness and material for insulator layer130, and other variations/techniques for achieving a reduction inquantum-mechanical tunnelling known in the art can be used to the extentthey are compatible with the present objectives.

For a preferred embodiment of this invention, polycrystalline silicon(poly-Si) is used as the material for gate-electrode 110. Other possibleembodiments may utilize alternative gate materials such aspolycrystalline silicon-germanium or metals, or any number of otherconventional materials.

An exemplary process for fabricating the NDR-MISFET in a conventionalCMOS fabrication facility is depicted in FIG. 4 A standard p-typesilicon starting substrate 120 is first processed through standardisolation-structure-formation process steps; the surface of substrate120 is then moderately doped (to ˜5×10¹⁸ cm⁻³) by a shallow Boronimplant. Subsequent to this a deposition of silicon dioxide (˜6 nm) isdone (or thermal oxidation) in a manner so that the Boron becomesincorporated into a gate insulator 130 near the surface of siliconsubstrate 120. The resultant dopant concentration in the Si channel nearthe surface is several times lower than it is directly after the implantstep above, due to segregation of Boron into gate insulator 130. Asnoted earlier, the Boron dopant then acts effectively as an electrontrap during operation of device 100. In contrast to some of the priorart implantation techniques discussed earlier, the oxidation stepappears to incorporate the Boron in a manner that facilitates shallowelectron traps, making it easier for charge to move in and out of gateinsulator 130.

Next, polycrystalline silicon is deposited and patterned to form gateelectrode 110. N-type dopant ions such as Arsenic are subsequentlyimplanted at moderate dose to form the lightly doped source/drainregions self-aligned to gate 110, after which sidewall spacers (notshown) are formed by conformal deposition and anisotropic etching of aninsulating layer such as silicon nitride. Deep source/drain contactregions 140 and 150 are then formed by ion implantation of Arsenic orPhosphorus and thermal annealing to activate the dopants. Devicefabrication is completed with standard passivation, contact andmetallization processes. While not explicitly shown, it is apparent,because only conventional processing is required, that other CMOSdevices can be formed in the same mask with the present NDR device 100,so that, for example, memory and logic circuits can be formed at thesame time as the present device, and thus integrated directly to form aconventional CMOS circuit having NDR capability. While the above isexplained with reference to a CMOS process, it will be appreciated bythose skilled in the art that other types of starting semiconductormaterials could also be used instead. Suitable and/or optimal processingconditions for achieving the NDR mode in any particular CMOS compatibleenvironment will be easily designed and determined by those skilled inthe art through conventional modelling and experimentation techniques.

As a final note it is preferable that during normal operation of device100 that a body contact (V_(B)) should be electrically biased (e.g. at afixed potential of 0 Volts, as is typical for n-channel MOSFETs). Ifbody terminal (V_(B)) is not connected (i.e. is “floating”) then the NDRbehavior is drastically diminished or even eliminated. This is becauseholes which are generated by hot electrons will accumulate at thechannel-to-source junction, forward biasing the junction and effectivelyreducing the transistor threshold voltage (counteracting thecharge-trapping effect of increasing the threshold voltage), if theholes are not allowed to flow out of the channel region through the bodycontact. Thus, if NDR-MISFET 100 is implemented in asilicon-on-insulator substrate, or in a thin film of polycrystallinesilicon, care must be taken to provide a body contact. This aspect ofthe invention can also be exploited of course for certain applications,where it may be potentially useful to be able to turn on or turn off theNDR mode by connecting or disconnecting (switching) a bias voltage tobody terminal V_(B), respectively.

With the prior art, even if a device exhibiting adequate negativedifferential resistance can be produced, it is still a daunting task tointegrate such a device into a conventional CMOS process. Since thedevice in this invention is inherently an NMOS structure, integration ofthis device with conventional logic CMOS devices is straightforward. Theillustrative flow in FIG. 4 allows an NDR device process module to becompletely de-coupled from a conventional process, to allow forindependent optimization of the NDR devices and the CMOS devices. Thismakes it more straightforward to scale the NDR device in this inventionwith future generations of CMOS integrated-circuit technology.

It will be apparent to those skilled in the art the aforementioned NDRdevice can be advantageously employed in both memory and logicapplications, and in the types of circuits as described in the prior artabove in references [1] through [10], i.e., as a memory device, as partof a logic circuit, a self-latching logic device, an amplifier, anoscillator, power management, and many other environments where itsuseful characteristics can be exploited.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. It will be clearly understood by those skilled in theart that foregoing description is merely by way of example and is not alimitation on the scope of the invention, which may be utilized in manytypes of integrated circuits made with conventional processingtechnologies. Various modifications and combinations of the illustrativeembodiments, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thedescription. Such modifications and combinations, of course, may useother features that are already known in lieu of or in addition to whatis disclosed herein. It is therefore intended that the appended claimsencompass any such modifications or embodiments. While such claims havebeen formulated based on the particular embodiments described herein, itshould be apparent the scope of the disclosure herein also applies toany novel and non-obvious feature (or combination thereof) disclosedexplicitly or implicitly to one of skill in the art, regardless ofwhether such relates to the claims as provided below, and whether or notit solves and/or mitigates all of the same technical problems describedaboved. Finally, the applicants further reserve the right to pursue newand/or additional claims directed to any such novel and non-obviousfeatures during the prosecution of the present application (and/or anyrelated applications).

What is claimed is:
 1. A method of forming a semiconductor transistordevice, including the steps of: incorporating a dynamically variablethreshold voltage in the semiconductor transistor device; providingcontrol terminals for the semiconductor transistor device, so that saiddynamically variable threshold voltage can be increased and/or decreasedrapidly in response to control signals received by the semiconductortransistor device and such that the semiconductor transistor device canbe operated in a negative differential resistance mode.
 2. A method offorming a semiconductor transistor device, including the steps of:providing three terminals for the semiconductor transistor device,including two voltage bias terminals and a control signal terminal;providing the semiconductor transistor device with a negativedifferential resistance (NDR) mode which can be invoked by suitablebiasing of said three terminals of the semiconductor transistor device,including (a): applying a voltage bias across said two voltage biasterminals sufficient to set up a current path between said two voltagebias terminals; and (b) applying a control signal to said control signalterminal for controlling conduction in said current path by controllinga density of charge carriers available in said current path.
 3. A methodof forming a semiconductor device comprising the steps of: (a) forming asilicon based semiconductor transistor; (b) configuring said siliconbased semiconductor transistor to operate with a negative differentialresistance mode; wherein both steps (a) and (b) are performed usingmetal oxide semiconductor processing operations.
 4. The method of claim1, further including a step: forming a charge trapping region adapted torapidly trap and de-trap charge carriers in the semiconductor transistordevice to effectuate said dynamically variable threshold voltage.
 5. Themethod of claim 1, wherein the semiconductor transistor device is formedas a silicon based negative differential resistance (NDR) capable fieldeffect transistor (FET), and further including a step of: forminganother metal oxide semiconductor device that does not include an NDRmode at the same time that the NDR capable FET is formed.
 6. The methodof claim 1 wherein said control signals are applied to a gate controlterminal, a source terminal and a drain terminal of the semiconductortransistor device.
 7. The method of claim 1, wherein an onset point forsaid negative differential resistance mode is adjusted by controllingone or more processing operations during manufacture of thesemiconductor transistor device.
 8. The method of claim 2, wherein anonset point for said negative differential resistance mode is adjustedby controlling one or more processing operations during manufacture ofthe semiconductor transistor device.
 9. The method of claim 8, whereinan onset point for said negative differential resistance mode isscaleable relative to a channel length used in the semiconductortransistor device.
 10. The method of claim 2, wherein said controlsignal is set to a predetermined constant value to enable said NDR mode,and said voltage bias is adjustable so that the semiconductor transistordevice operates as a normal field effect transistor (FET) in oneoperating region, and with an NDR mode in a second operating region. 11.The method of claim 2, further including a step of: providing a bodycontact terminal of the semiconductor transistor device that is set to afixed potential.
 12. The method of claim 3, wherein said NDR modeachieved through charge trapping and charge de-trapping.
 13. The methodof claim 3, wherein said NDR mode is not achieved through a band to bandtunneling mechanism.
 14. The method of claim 3, wherein said siliconbased semiconductor transistor is a field effect transistor (FET) thatbehaves both like an NDR device and a non-NDR device.
 15. The method ofclaim 14, wherein said NDR mode is achieved in said FET by biasing thedevice to have a variable threshold voltage.
 16. The method of claim 3,wherein the semiconductor transistor device is formed as a negativedifferential resistance (NDR) capable field effect transistor (FET), andfurther including a step of: forming another metal oxide semiconductordevice that does not include an NDR mode with said metal oxidesemiconductor processing operations when the NDR capable FET is formed.17. The method of claim 3, wherein an onset point for said negativedifferential resistance mode is adjusted by controlling one or more ofsaid MOS processing operations.
 18. The method of claim 17, wherein anonset point for said negative differential resistance mode is scaleablerelative to a channel length used in the semiconductor transistordevice.
 19. The method of claim 3, wherein (a) a control signal appliedto a gate of said silicon based semiconductor transistor is set to apredetermined constant value to enable said NDR mode; and (b) anadjustable voltage bias is applied to source and drain terminals of thesilicon based semiconductor transistor so that it operates as a normalfield effect transistor (FET) in one operating region, and with an NDRmode in a second operating region.
 20. The method of claim 3, furtherincluding a step of: providing a body contact terminal of said siliconbased semiconductor transistor that is set to a fixed potential.